This invention relates to a process for completing or working over a well penetrating a subterranean formation such as an oil and/or gas producing formation.
By definition a completion or workover fluid is a fluid that is placed against the producing formation while conducting such operations as well killing, cleaning out, drilling in, plugging back, controlling sand, or perforating. Basic fluid functions are to facilitate movement of treating fluids to a particular point downhole, to remove solids from the well, and to control formation pressures.
Required fluid properties vary depending on the operation, but the possibility of formation damage is always an important concern. In certain operations, such as gravel packing or perforating, sand-face or perforation plugging is a prime concern. In recent years many new fluid systems have appeared, most due to the recognition of the high risk of reducing the productivity, or completely plugging certain sections of the producing zone, through contact with a foreign fluid.
Fluid density should be no higher than needed to control formation pressure. With reasonable precautions a hydrostatic pressure of 100-200 psi over formation pressure is adequate. Balanced or slightly underbalanced pressure workovers are ideal from the standpoint of formation damage and, with proper equipment to contain the surface pressure, are practical for some operations.
Fluid loss characteristics of the treatment fluid are tailored to prevent loss of excessive quantities of fluid to the formation, or to permit application of "hydraulic stress" to an unconsolidated sand formation. Bridging at the formation face by properly sized acid-soluble particles (calcium carbonate) is a conventional approach to fluid loss control. In some cases, oil soluble resin particles have been used in place of calcium carbonate. In either case colloidal particles are also required for an effective seal.
Viscosity-related characteristics, such as yield point, plastic viscosity, and gel strength, can be tailored to provide fluid lifting capacity required to bring sand to the surface at reasonable circulating rates. In some cases viscosity builders cause permanent reduction in permeability. This can be minimized by careful polymer selection along with adequate fluid loss control to limit invasion.
In a typical treatment fluid, a fluid viscosity builder is provided to control fluid loss. Viscosity builders such as hydroxyethyl cellulose or other hydrolyzable polymer are commonly used, and typically result in formation of a gel filter pad covering all or part of the treated formation. When a solid fluid loss additive such as particles of calcium carbonate is included in the fluid, it is generally necessary to carry out an acid treatment to dissolve the particles and to restore formation permeability.
Unbroken hydroxyethyl cellulose gel, even in the absence of calcium carbonate particles, can cause significant permeability reduction even after backflow. The possibility of permanent permeability loss from invasion of viscosity builders into the formation dictates that proper bridging particles should be used. Unfortunately the most commonly used material, calcium carbonate, must be removed after the treatment by acidizing. Even with acidizing, some permanent damage is possible. The acid treatment also results in breaking of the gel in the gel filter pad, but in some cases additional gel-breaking treatment is required.